In the production of a large variety of industrial products, it is necessary to weld metal parts together along a relatively long seam or joint. The seams may be formed, for example, where parts of an automobile or truck frame are butted together, where large metal plates are overlapped, or at butt joints formed between metal plates, as in the plates forming the hulls of ships or large pressure vessels. Most commonly, such welding has been done with manually operated arc welders or by semi-automatic welding machines that carry an arc welding head but are guided along the seam or joint by an operator.
Various automatic welding systems have been developed in an effort to automate such welding procedures. A fully automated system, which does not require substantial intervention by a human operator, must be capable of determining the position of the seam and must be able to guide the welding head along this seam. For some types of parts to be welded together, the location of the seam from part to part is very consistent, and it may then be possible to use standard numerically controlled preprogramming techniques to define the position of this seam and to guide the welding head along it. However, it is more typical in production situations for the position of the seam to vary from part to part, either because of tolerance variations in the parts themselves or in the positioning of the part in its fixture, with the result that guidance of the welding head by standard numerical control programming is not feasible.
Thus, to truly automate the typical seam welding operation, it is necessary for the welding machine to be capable of sensing the position of the seam and to use the information so determined to guide a welding head in proper position to accurately deposit the weld metal in the seam. A variety of approaches have been proposed for sensing the seam including magnetic sensors, optical sensors, electrical resistivity sensors which sense the current flowing through the arc welding rod, and tactile sensing probes. The machine must then utilize the data provided from the sensor to track the sensor accurately along the seam, to determine the position of the sensor when it is tracking the seam, and to use the position information from the sensor to position the welding head to deposit metal at the right spot in the seam. Desirably, the welding machine should be able to define the location of the seam with precision and guide and orient the welding head to deposit the weld with commensurate precision, all at a relatively high speed, without operator intervention, and without loss of capability when welding complex non-linear seam contours which may vary from work piece to work piece.